"We have plenty of challenges ahead, but this launch went so well, we're delighted," said JPL's Pete Theisinger, project manager for the Mars Exploration Rover (Mer) missions.

The American missions follow quickly behind that of Europe's Mars Express and Beagle 2 expedition to the Red Planet.

The European space craft, now more than one million kilometres from the Earth, left the Russian cosmodrome in Baikonur, Kazakhstan a week ago.

Over the next few months Mars and the Earth will be closer than at any time in recorded history, cutting the journey to less than seven months.

Martian record

The US rovers are going to Mars to study its geology, seeking chemical signatures that would confirm that abundant water once existed on the planet's surface.

"The instrumentation on board these rovers, combined with their great mobility, will offer a totally new view of Mars, including a microscopic view inside rocks for the first time," Ed Weiler, Nasa headquarters associate administrator for space science, said earlier this week.

"However, missions to Mars have proven to be far more hazardous than missions to other planets.

"Historically, two out of three missions, from all countries who have tried to land on Mars, ended in failure.

"We have done everything we can to ensure our rovers have the best chance of success," Dr Weiler said.

Spirit is due to arrive on the Red Planet in the New Year, a few days after the Beagle 2 probe. It will be joined by the second Nasa rover at the end of January.

Water clues

Only three Nasa space craft have ever touched down successfully on Mars before: the Viking landers of 1976 and Mars Pathfinder.

If all goes well, the rovers will head for two places that appear to have once been flooded by water.